uxcell 4mm Inductive Proximity Sensor Switch Detector NPN NO DC 5-30V 200mA 3-Wire SN04-N for 3D Printer Probe Bed Leveling,MCU Board: Amazon.com: Industrial & Scientific
DC NPN 4-Wire NO NC Nomal Open/Close Proximity Sensors 4m Body M12 : Industrial & Scientific - Amazon.com
Amazon.com: Inductive Sensor, Inductive NO NPN Sensor, for Security Equipment : Industrial & Scientific